The present mainstream of liquid crystal display elements are those utilizing nematic liquid crystals. Liquid crystal display elements presently in practical use include a TN element twisted by 90.degree., an STN element usually twisted by 180.degree. or more, and a TFT liquid crystal display element utilizing thin film transistors. In addition to these, there have been developed liquid crystal display elements of various driving systems, such as a lateral-electric-field-type liquid crystal display elements of the IPS (In Plane Switching) mode having improved view-angle properties. The progress of liquid crystal display elements is not limited to only these modes, but active efforts to improve peripheral materials have been made in order to attain improvement of the properties of liquid crystal display elements.
With wider use of liquid crystal display elements in various fields, liquid crystal display elements having improved properties have been demanded. Such demands include demands in relation to the alignment properties of liquid crystals represented by the pre-tilt angle, demands in relation to the electrical properties of liquid crystal display elements such as current consumption, voltage holding ratio, and residual voltage, and demands for the reliability of such properties in long-time use.
Of these properties, the required pre-tilt angles of liquid crystals differ depending on the driving systems of liquid crystal display elements. For example, TN elements or TFT elements, in which liquid crystals are twisted by 90.degree., require a pre-tilt angle of 1 to 6.degree., and STN elements having larger twist angles require a pre-tilt angle of 3 to 8.degree.. In addition to pre-tilt angles, properties related to the alignment of liquid crystals such as alignment uniformity, alignment stability, and anchoring energy at the liquid crystal-aligning film interface are also important, because these properties influence the performance of liquid crystal display elements. In addition, the process margin of these properties in the manufacture of liquid crystal display elements is also important. A significant problem will arise if the pre-tilt angle or alignment of liquid crystals varies depending on the conditions of drying solvent after the application of alignment materials, conditions of imidizing the polyamic acid (typically by heat-treatment), or conditions of annealing after injecting liquid crystals.
For STN liquid crystal display elements, especially those of a low-voltage type used in portable devices, low current consumption is demanded because of the low driving voltage of liquid crystal display elements. That is, since voltage applied to liquid crystals lowers accordingly when the current consumption of a liquid crystal element rises, the rise of liquid crystal molecules becomes insufficient, and contrast lowers. For liquid crystal display elements of a low-voltage type, change in current consumption experienced in long-time use (reliability) is also important. Since STN display elements use a slight potential difference for turning a display on or off, if the current consumption of an element changes, voltage applied to liquid crystals also changes making normal driving impossible. In an extreme case, there results a phenomenon such that no images of a liquid crystal display element are displayed when the element is driven for a long time.
For TFT liquid crystal display elements, on the other hand, requirements for voltage holding ratio and residual voltage are particularly important. If voltage holding ratio is small, voltage applied to liquid crystals during the field period lowers, resulting in low contrast. If residual voltage is high, electric charge remains even if the voltage is turned off after the voltage is impressed, and images that should be erased remain as residual image. In TFT liquid crystal display elements, the residual image phenomenon is one of the most critical problems.
The object of the present invention is to provide a liquid crystal aligning film for obtaining a liquid display element having an optimal pre-tilt angle, low current consumption, and highly reliable pre-tilt angle and current consumption even after long-term use.
With regard to an example of a liquid crystal alignment material having stable liquid crystal alignment properties and pre-tilt angles and good electro-optical, properties, Japanese Patent Application Laid Open No. 07-120768 discloses an alignment material which contains a polyamic acid having an aliphatic tetracarboxylic dianhydride as an essential component, and a polyamic acid having an aromatic tetracarboxylic dianhydride as another essential component.
However, since no amine components having groups that increase the pre-tilt angle of liquid crystals are described in the cited reference, obtaining an adequate pre-tilt angle is difficult. That is, it is difficult to obtain a material concurrently having an adequate pre-tilt angle, excellent electrical properties, and the reliability of these properties through the use of the process disclosed in the cited reference. Although the cited reference describes that a siloxane-based diamine is preferred, it has a small pre-tilt angle, as well as a problem of liquid crystal alignment (see Comparative Examples described herein).